Well the first time I ever had this happen, the first game I ever had this problem with, was when I installed Fable: The Lost Chapters; that's when I noticed it started happening to other games as well. It sucks, because I play gametap, and it seems to have affected almost every gametap game I have. I've uninstalled Fable: The Lost Chapters, but the problem is still there, so maybe it isn't the cause.
EDIT: Plus why would a game affect any other games? A microsoft game at that. Ugh, this sucks.
I'm thinking of reformatting sometime soon, and hopefully the problem will be gone, but reformatting is a pain in the ass.